LCD units generally include a liquid crystal (LC) cell including a LC layer and a pair of transparent substrates sandwiching therebetween the LC layer, the LC cell defining an array of pixels for display of an image.
LCD units are categorized into a transmissive LCD unit and a reflective LCD unit. In general, the transmissive LCD unit includes the backlight source, and controls the transmission of the light from the backlight source to display an image. The reflective LCD unit includes a reflection film that reflects the external light, and uses the light reflected by the reflection film as a light source for the display of an image. The reflective LCD unit, that does not need the backlight source, is superior to the transmissive LCD unit in the view point of low power dissipation, thin thickness and light weight. However, the reflective LCD unit has the weakness that visibility of the screen is lowered in a dark environment because the reflective LCD unit uses the ambient light as the light source for the display of an image.
A transflective LCD unit is known as a LCD unit that has the advantage of both the transmissive LCD unit and the reflective LCD unit (for example, refer to Patent Publication-1). The transflective LCD unit includes a transmissive area and a reflective area in each pixel of the LCD unit. The transmissive area transmits therethrough the light emitted from the backlight source, to use the backlight source as the display light source. The reflective area includes a reflection film that reflects the external light and uses the reflected light as the display light source. The transflective LCD unit turns OFF the backlight source in a bright environment, and uses the reflective area to display an image to save the power. The transflective LCD unit turns ON the backlight source in a dark environment, and uses the transmissive area to display an image even in the dark environment.
Some LCD units operate in a lateral-electric-field drive mode, such as an in-plane-switching (IPS) mode or fringe-electric-field drive mode, such as a fringe-field-switching (FFS) mode. The IPS-mode LCD unit includes in each pixel a pixel electrode and a common electrode, that are formed on the same substrate to apply a lateral electric field to the LC layer The lateral-electric-field mode LCD unit realizes a wider viewing angle compared to a twisted-nematic(TN)-mode LCD unit, by rotating LC molecules in the LC layer in the direction parallel to the substrate surface for the display of an image.    The publications cited in this text include:    Patent Publication-1 (JP-2003-344837A);    Patent Publication-2 (JP-2006-017136A);    Patent Publication-3 (JP-2007-041572A);    Patent Publication-4 (JP-2003-140190A);    Patent Publication-5 (JP-2007-071938A).    Patent Publication-6 (JP-2003-270627A); and    Patent Publication-7 (JP-2007-199340A).
Patent Publication-2 describes an example of the lateral-electric-field drive mode LCD unit that includes both the transmissive area and the reflective area, i.e., transflective LCD unit. The transflective LCD unit described in Patent Publication-2 includes in the transmissive area a LC layer having a retardation of λ/2 for a light having a wavelength“λ” of 550 nm, and a λ/2 retardation film and a LC layer having a retardation of λ/4 in the reflective area. The transflective LCD unit drives the LC layer in a normally black mode. Compared to the transmission characteristic of the conventional transflective LCD unit operating in a TN mode or electrically-controlled birefringence (ECB) mode, the LCD unit operating in the lateral-electric-field mode in the transmissive area is known to have a higher viewing angle characteristic and thus be superior in the image quality.
However, in the transflective LCD unit described in Patent Publication-2, it is needed to reduce the cell gap of the LC layer in the reflective area. It is known that the threshold electric field (Ec) of the lateral electric field is generally expressed by the following formula:
                              E          c                =                              π            d                    ⁢                                                    K                22                                                              ɛ                  0                                ⁢                                  ɛ                  a                                                                                        (        1        )            
where EC, d, K22, and ∈ are the threshold electric field, cell gap that is the thickness of the LC layer, elastic coefficient and dielectric constant, respectively. From the above formula, it is understood that the smaller cell gap requires a larger threshold electric field (or a drive voltage of LC), and thus requires a smaller distance between the pixel electrode and the common electrode. This is because the electric field is inversely proportional to the distance between the pixel electrode and the common electrode and is proportional to the potential difference between the pixel electrode and the common electrode. The pixel electrode and common electrode are each generally configured by a comb-teeth electrode. The narrow distance between the comb-teeth electrodes reduces the area that contributes to the reflection of light to thereby reduce the reflectance of the reflective area. In the case where the ratio of the width of the comb-teeth electrodes to the distance between the comb-teeth electrodes is 2:3, for example, the area that contributes to the reflection of light is ⅗ of the total reflective area. For a larger reflective area, it is necessary to increase the ratio of the reflective area to the transmissive area in a pixel, which results in reduction of the transmissive area and thus reduces the image quality of the transmissive area.
Patent Publication-3 describes a solution to the problem of the reduced reflectance of the transflective LCD unit in Patent Publication-2. In the transflective LCD unit described in Patent Publication-3, the transmissive area includes a LC layer having a retardation of λ/2 without the λ/2 retardation film whereas the reflective area includes a LC layer having a retardation of λ/4. In addition, both the reflective area and transmissive area of each pixel include respective TFTs (thin-film-transistors) for connection between the data signal line and the corresponding pixel electrode, and respective common electrodes. Further, it is disclosed as an example that the reflective area and transmissive area are driven by drive signals having an inverted ON-OFF relationship therebetween. The inverted ON-OFF relationship of the drive signals is such that one of the drive signals is ON or active during the OFF period or inactive period of the other of the drive signals, and vice versa. The term inverted drive signals or inverted ON-OFF drive signals as used in this specification refer to drive signals having the inverted ON-OFF relationship therebetween.
The technique of Patent Publication-3 discloses that, in the absence of the applied voltage(driving signal voltage), the transmissive area operates a normally black mode wherein absence of the applied voltage provides a black state or a dark state, and the reflective area operates in a normally white mode wherein the reflective area assumes a bright state or white state in the absence of the applied voltage, whereby the entire reflective area can be used as the reflection film. This provides high reflectance for the reflective area.
However, in the technique of Patent Publication-3, since the transmissive area operates in the normally black mode whereas the transmissive area operates in the normally white mode, and accordingly, both the areas are driven by the inverted ON-OFF drive signals for display of the same image, there occurs leakage of an electric field beyond the interface between the reflective area and the transmissive area in the pixel. The electric field occurring in this state is the same as the electric field occurring in the ON state of the drive signal. This electric field involves substantially no problem upon display of a bright state because the change of transmitted light is extremely small compared to the brightness provided by the drive signals upon display of the bright state. On the other hand, when the transmissive area is driven in the normally black mode for display of the dark state, a strong electric field occurs at the boundary area between the reflective area and the transmissive area. This electric field changes orientation of LC molecules, to change the polarization of the light transmitted through the interface, and thus generates leakage of light, which degrades a contrast ratio of the LCD unit. A light shield film is thus provided partially on the counter substrate for suppressing the leakage light at the boundary area.
However, in the LCD unit described in Patent Publication-3, unlike the case of Patent Publication-1 where both the transmissive area and reflective area are driven in the normally black, there may be a reflected light reflected by the shield film if the light emitted from the backlight source passes in a slant direction through the position where the orientation of the LC molecules in the reflective area is changed, or if the orientation of LC molecules in the reflective area is changed upon display of the dark state in the presence of the applied voltage. This reflected light may be further reflected by the reflection film to cause a leakage light. In the LCD unit of Patent Publication-3, due to the normally white mode of the reflection area, the reflectance of the reflective area is increased because all the area of the reflection film can be used for reflection. However, for prevention of the leakage light in the slant direction at the interface, a shield film having a larger width than an ordinary shield film is needed. This causes the problem of reduction in the effective opening ratio of the pixel by an increased width of the shield film The effective opening ratio as used herein refers to a ratio of the effective opening area of the pixel passing therethrough the light to the total pixel area.
Patent Publications-4 and -5 describe provision of a shield film in the boundary area for prevention of the leakage light generated from the backlight source due to the disturbance of orientation of the LC molecules caused by a slope area that adjusts the thickness of the LC layer at the step difference between the transmissive area and the reflective area. In this case, the shield film cannot prevent leakage of the re-reflected light generated at a sloped portion or stepped portion at the boundary area formed by the step difference or generated in the reflective area. For example, it is disclosed in the technique of Patent Publication-5 that the leakage light caused by the disturbance of the orientation of LC molecules due to the step difference between the reflective area and the transmissive area can be suppressed at the boundary by a shield film on the counter substrate. However, in this publication, both the reflective area and transmissive area are covered by a common transparent electrode, and the influence by occurring of the electric filed at the boundary area between the reflective area and the transmissive area is not considered. Therefore, the effect of the suppression is limited for the different drive scheme as used in the technique of Patent Publication-2. The drive scheme of Patent Publication-2 causes the disturbance of orientation of LC molecules not only in the vicinity of the TFT (thin-film-transistor) substrate but also in the entire cell gap of the LC layer in the boundary area. That is, only the shield function provided in the vicinity of the TFT substrate cannot well suppress the leakage light. In addition, the external light incident from the front side of the LCD unit is reflected by the shield film formed as a metallic film on the TFT substrate, to thereby degrade the image quality of the LCD unit. The term “boundary area” as used above refers to the area including the boundary between the reflective area and the transmissive area and the vicinity of the boundary.
Patent Publication-6 discloses a structure wherein a shield film is formed on at least one of the TFT substrate and counter substrate in the boundary area between the reflective area and the transmissive area. However, similarly to Patent Publications-4 and -5, both the reflective area and transmissive area are covered by a common transparent electrode and connected together in this publication, whereby an electric filed is not generated in the boundary area between the transmissive area and the reflective area. Thus, the influence caused by occurring of the electric field is not considered in this publication, and thus the slope area, in which a thickness adjustment layer is formed between the reflective area and the transmissive area, is shielded by a shield film as the boundary area. Therefore, the effect of the suppression is limited for the different drive scheme as used in the technique of Patent Publication-2. In addition, the light incident from the front side of the LCD unit may cause degradation of the image quality due to the influence by the disturbance of the orientation of the LC molecules unless the boundary area between the transmissive area and the reflective area is shielded.
Patent Publication-7 discloses a structure of shielding between adjacent pixels. In this structure, a shield film is provided on the TFT substrate between adjacent pixel electrodes, and another shield film is also provided on the counter substrate. The another shield film on the counter substrate has a width smaller than the gap between the reflective electrode of one of adjacent pixels and the reflective electrode of the other of the adjacent pixels. If operation of this LCD unit in Patent Publication-7 is similar to that described in Patent Publication-2, the backlight incident onto the transmissive area cannot be shielded by the shield film on the counter substrate, to degrade the image quality. At the same time, the light incident from the front side of the LCD unit may also cause degradation of the image quality due to scattering by the disturbance of the orientation of the LC molecules in the boundary area and reflection by the shield film formed on the TFT substrate. In the structure such as described in Patent Publications-4 to -7, the drive electrodes of the reflective area and drive electrode in each pixel are connected together to have the same potential. Thus, the influence by the electric field in the boundary area between the reflective area and the transmissive area is not considered, although there is some concern as to the disturbance of the orientation of the LC molecules only in the slope area due to the thickness adjustment layer provided in the boundary area. However, the effect of this shield structure is limited for the different drive scheme as used in the technique of Patent Publication-2, so long as the conventional configuration is considered.